When it comes to meeting a building's air conditioning requirements, certain times are better than others. In fact, you might say that the difference is like day and night - literally.

Without question, air conditioning during summer daytime hours is the largest single contributor to utility "peak demand" charges. After noon, as more air conditioners are required to maintain comfortable temperatures, the increased call for electricity adds to the high demand already created by lighting, office equipment, computers and other electrical devices. This requires the utility to bring additional, more costly generating sources on line to handle the increase. Consequently, commercial users whose large air conditioning loads contribute to these added generating requirements are assessed an additional charge based on their highest on-peak demand for electricity.

The obvious solution is to shift as much as possible of the electrical usage to the nighttime, off-peak hours, when a building's electrical needs are at a minimum. This is the theory behind the Ice Bank® Stored Cooling System from CALMAC® Manufacturing Corporation.

[an error occurred while processing this directive]The system not only significantly lowers demand charges during air-conditioning season but also decreases total energy usage, using a standard packaged chiller to produce solid ice at night during off-peak periods. This also takes advantage of the utility's underutilized generating capacity at night and lower customer rates. The ice is built and stored in modular ice tanks to provide cooling to help meet the building's air conditioning load requirement the following day.

This kind of innovation is nothing new for CALMAC Manufacturing Corporation, the heating and air conditioning innovator best-known as the market leader in the thermal energy storage industry. In fact, CALMAC's Ice Bank product line accounts for 50% of all installations in the United States and 75% of the export market.

Founded in 1947 in Englewood, New Jersey, CALMAC has over 2,000 ice storage installations worldwide in over 30 countries on six continents. Installations range from 30 ton-hr. capacity required by a McDonald's franchise in Geneva, Switzerland, to the 30,000 ton-hr. needed for the J.C. Penney headquarters in Plano, Texas. Their largest international installation is located in the T&C Building in Kaohsiung, Taiwan, the 10th tallest building in the world. The company is also one of the world's largest manufacturers of ice making equipment for skating rinks.

Clearly, CALMAC could not have achieved such impressive credentials unless its Ice Bank system were of superior quality. Ironically, although the system's quality was unchallenged, the pressure transmitters used in the system were beginning to become a real trouble spot.

"We had been using competitive transmitters for some time, but we grew increasingly dissatisfied with their performance," said CALMAC President Mark MacCracken. "They were extremely sensitive and would go out of calibration regularly. In fact, it happened quite frequently during shipping. And they were difficult to adjust."

Consequently, CALMAC decided to utilize pressure transmitters from Setra Systems, Inc., of Boxborough, Massachusetts, a leading designer and manufacturer of highly accurate capacitive-based pressure sensors. The Setra System's Model C264, which CALMAC elected to use, senses differential or gage (static) pressure and converts this pressure difference to a proportional electrical output for either unidirectional or bidirectional pressure ranges.

To fully understand the critical role that the transmitters play in the Ice Bank system, it is necessary to have a basic understanding of the system itself. The Ice Bank system is a modular insulated polyethylene tank containing a spiral-wound plastic tube heat exchanger surrounded with water. At night, a 75% water/25% glycol solution from a standard packaged air conditioning chiller circulates through the heat exchanger and extracts heat until eventually about 95% of the water in the tank is frozen solid. The fluid is at 25° F and freezes the water surrounding the heat exchanger.

The following day, the stored ice cools the solution from 52° F to 36° F. A temperature modulating valve set at 44° F in a bypass loop around the tank permits a sufficient quantity of 52° F fluid to bypass the tank, mix with the 36° F fluid, and achieve the desired 44° F temperature. The 44° F fluid enters the coil, where it cools air typically from 75° F to 55° F. The fluid leaves the coil at 60° F, enters the chiller and is cooled to 52° F.

The role of the Setra Model C264 pressure transmitters is to monitor the change in water level created by the forming of ice. The pressure transmitter is connected to one of two pneumatic tubes joined by a "Y" connector to a single probe inserted into the water. (The pressure transmitter is never in direct contact with the water.) A small continuous volume of air is pumped through the probe, producing air bubbles out the bottom. Expanding ice in the tank causes the surrounding water to rise. The water level rises above the low point of the probe and the air pressure inside the probe increases. The Model C264 senses this pressure and produces a proportional output. The signal produced by the output is sent to a controller where the pressure reading is displayed. This change in pressure indicates the percentage of ice in the tank.

The impact of the Setra transmitters on the Ice Bank product line has been significant, according to MacCracken.

"We have had far less callbacks on our products; in fact, we have had none," he said. "Obviously, the quality of our transmitters now is far superior to anything we have had before." 

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